A fuel vapor treatment apparatus includes a canister that temporarily adsorbs fuel vapor evaporated in a fuel tank. The fuel vapor desorbed from the canister is introduced and purged, through a purge passage, into an intake passage of the internal combustion engine by intake pressure. A purge control valve disposed in the purge passage controls amount of air including the fuel vapor, which is purged into the intake passage.
The purged fuel vapor is combusted in the engine with the fuel supplied from a fuel injector. Hence, it is necessary to accurately measure the amount of fuel vapor in order to keep an air-fuel ratio within a predetermined range.
JP-5-18326A and JP-6-101534A show a system in which concentration of the fuel vapor is detected in order to control the opening degree of the purge valve and the fuel injection amount in such a manner as to obtain a target air-fuel ratio. However, even if the opening degree of the purge valve is controlled based on a preset flowrate characteristic in order to obtain the target air-fuel ratio, the amount of fuel vapor purged from the canister may be different from the target value due to a dispersion of the flowrate characteristic of each purge valve or a varying of the flowrate characteristic with age. As the result, an actual air-fuel ratio of the engine may deviate from the target air-fuel ratio.
Besides, when the measured concentration of the fuel vapor deviates from the actual concentration of the fuel vapor, the actual air-fuel ratio may deviate from the target air-fuel ratio.